1. Field of the Invention
The invention pertains to automatic light-level control in low-light-level television cameras (LLL/TV) and more particularly to the automatic control of LLL/TV cameras operating in light levels variable from starlight to full daylight.
2. Description of the Prior Art
Automatic light-level control in LLL/TV cameras is a special problem, because the dynamic range of light-levels in nature and the dynamic range of camera sensors differ by six orders of magnitude. Since the development of LLL/TV in the mid-1960's the dual problems of photocathode protection and image quality over a wide dynamic range of light levels has existed. A camera devised with a sensitivity that allows image perception by starlight (approximately 10.sup.-5 lux) is expected by users to be functional at any light level. Typically, however, the sensors involved begin to saturate and produce poor images at light levels 100 times above maximum sensitivity. Additionally, the sensors incur permanent damage at light levels above 10.sup.-2 lux.
A light level control that permits use of LLL/TV sensors at all light levels automatically has been needed from the advent of LLL/TV in 1963. Many light control systems have been developed to provide automatic light control for LLL/TV cameras. Most of these systems, however, have exhibited limited dynamic range, insufficient operating speed, low reliability, or extreme complexity. One device of the prior art utilizes a motor driven servo to control a lens iris. The aperture size that may practically be provided by an iris is between F/0.9 to F/32, which represents a dynamic range of 1200 to 1, a range that is inadequate for light level variations from starlight to maximum daylight, a range of 10.sup.9 to 1. Additionally, a motor driven iris is too slow and does not exhibit sufficient reliability for use in environments where light levels change rapidly.
An attempt to provide a more suitable motor driven iris utilizes a neutral density filter spot within the center of the iris. This configuration permits a more rapid variation of the lens aperture, for as the iris closes down more of the aperture is occupied by the spot. Though the dynamic range is increased to 10.sup.6 -to-1 it is still inadequate and the speed problems still remain, while additional optical problems are introduced. This technique has been utilized with stepped neutral density filters with which dynamic range requirements may be met. These filters, however, are expensive and the stepped functions between filters introduce severe control problems and increase reliability problems due to the additional mechanical complexity.
The best solution to the problem implemented in the prior art utilizes neutral density wedge filters which close down and open the aperture in accordance with the incident light level. This system exhibits a dynamic range of 10.sup.8 -to-1 and the speed of the aperture size variation is adequate. The assembly, however, of such a system is complex, large, and requires expensive optical components.
Though limited in dynamic range, pulse-width modulated gating of the light input by means of an electronic shutter is the most satisfactory of the prior art systems. Such a system provides 10.sup.6 -to-1 dynamic range with acceptable speed, reliability, and cost.
As previously indicated, the light level incident to a LLL/TV sensor may be controlled by either controlling the total light level or by modulating the duty factor of the light entering the sensor. Most prior art systems utilize total light level control, which introduce undesirable optical problems and exhibit speed and reliability deficiencies. Additionally, these systems function by changing the total incoming light level to the sensor and may, therefore, smear a moving scene during the frame time of the camera. Though many of these deficiencies are overcome with the utilization of a pulse-width modulating gating as described in U.S. Pat. No. 4,202,014 having the same inventor and assignee as the present invention, these systems do not provide sufficient dynamic range to permit operation of the LLL/TV cameras in environments that vary from starlight to daylight.